Technical Abstract:
Tylosin is an antimicrobial drug approved for use in broiler feed at sub-therapeutic levels for purposes of growth promotion. There is controversy about whether such use of antimicrobials could lead to the development of drug resistant pathogenic bacteria. Erythromycin is often the drug of choice for treating humans with campylobacteriosis. Both tylosin and erythromycin are classified as macrolide drugs and cross resistance between these antimicrobials may occur if either is used. Commercial broiler chicks were placed in isolation grow-out chambers and colonized with Campylobacter jejuni. At 14 days of age broilers began to receive a diet including 20 g tylosin phosphate per ton which was continued ad libitim for the rest of grow-out. Control broilers received the same diet with no added drugs. At 42 days of age, broilers were processed in a pilot plant with equipment which very closely models commercial conditions. Carcass rinses were collected after feather removal, after inside/outside washing and after immersion chilling. Campylobacter numbers after feather removal were not different according to feed type (3.53 log cfu per ml rinse for control carcasses and 3.60 for those fed medicated feed). Likewise, medicated feed did not affect Campylobacter numbers on carcasses after inside/outside washing (3.11 and 3.07 log cfu per ml rinse). Carcasses of broilers fed medicated feed had lower numbers of Campylobacter after chilling (1.45 log cfu/ml rinse) compared to control carcasses (2.31 log cfu/ml rinse). No Campylobacter isolated from control carcasses were resistant to erythromycin. However, all Campylobacter recovered from carcasses fed medicated feed were resistant to erythromycin (minimum inhibitory concentration of greater than 8 'g/ml). Application of tylosin in feed can act to promote growth and results in lower Campylobacter numbers on chilled carcasses; however, the Campylobacter that do remain are resistant to erythromycin.